1. Field of the invention
The present invention relates to a rotary recording apparatus such as a hard disk drive (hereinafter referred to as xe2x80x9cHDDxe2x80x9d) and to a method of controlling the apparatus. More particularly, the present invention relates to a technique effectively used to improve the head seek performance of a model following control system used for head position control in HDDs.
2. Background of the Invention
As a method of controlling the position of a head in a rotary recording apparatus such as an HDD, one-degree-of-freedom control methods using only feedback control and two-degree-of-freedom control methods in which a feedforward signal is added to a feedback signal are known. In two-degree-of-freedom control, a step response characteristic (seek performance) is achieved chiefly by means of a feedforward signal while a disturbance response characteristic (following performance) is achieved chiefly by means of a feedback signal. The two characteristics can therefore be optimized separately from each other to improve the control performance or response performance. For this reason, there are now an increasing number of cases of adopting two-degree-of-freedom control methods for HDD head position control, of which a reduction in head seek time is strongly required. As two-degree-of-freedom control methods, target-trajectory-generation filter method, feedforward compensation method, model following method, (called model reference methods in some cases) and other various control methods are known.
A target-trajectory-generation filter control method is a method in which a target position of a feedback loop is changed by a feedforward signal each time sampling is made for a control signal (in a control cycle). This control method achieves head position control at a higher speed and in an improved manner because a target position is changed by a feed forward signal for each sample. This method, however,has the drawback that it is difficult to generate an optimum feedforward signal because the head response characteristics (seek operation characteristics) depend on the characteristics of the feedback loop as well as on the feedforward signal.
A feedforward compensation control method is a method of feeding forward an acceleration signal to a conventional speed control loop. For example, a target speed is differentiated at the time of deceleration to calculate a target acceleration, which is applied as a feedforward signal to the control loop. In this method, various techniques for optimizing the feedforward signal can be devised, for example, the gain set for the feedforward signal can be determined on the basis of an adaptive control rule or a learning rule. However, mode change from speed control to position control is required, which cannot be smoothly performed, and it is difficult to optimize a response characteristic of settling.
A model following control method is a method using a system which has generally an internal control model obtained by modeling a control object, which gives the object a control output and a target trajectory such that control of the model is optimized. That is, in model following control, the same control target is input to a control model and a control object (feedforward control). If the model is perfect, the control model and the control object output equal control values. Generally, however, other factors, i.e., a disturbance, etc., exist, which hinder convergence to the target trajectory. Therefore feedback to the control object is provided (feedback control), thereby enabling seek with the head and following of the head to be controlled separately by feedforward control and by feedback control, respectively.
The model following control method is free from the above-described drawbacks of the target-trajectory-generation filter control method and the feedforward compensation control method, and has the following advantages, which the other two methods lack. That is, if a control object and a control model are equivalent to each other in terms of control characteristics (that is, there is no modeling error), the control object can be controlled in an optimum fashion theoretically. Even if there is a modeling error, a steady-state error in head position caused by the modeling error can be eliminated by the feedback loop. Further, a step response characteristic (seek operation characteristic) can be determined without being influenced by the feedback loop. Enabling determination of a step response characteristic with no influence from the feedback loop is particularly advantageous in a case where, as in the case of an HDD, there is a need for a high-speed seek operation while suppressing vibration due to mechanical resonance in a high-frequency range associated with the seek operation. That is, the model following control method makes it easier to obtain a response characteristic realizing high-speed seek operation while suppressing mechanical vibration of the head. Theoretically, it enables optimization of HDD head control in this manner.
For example, a control model using a state estimator may be mentioned. A control method has been employed in which a position, a speed and a acceleration are estimated by state estimators and respectively combined with a feedforward signal.
As measures for compensating a non-coincidence (modeling error) between a control model and an actual control object, techniques described below are known. For example, in the specification of U.S. Pat. No. 6,031,684, a technique of determining a change in the gain of an actuator during seek operation and effecting asymptotic conversion of the gain to an optimum value is described. A method of compensating only a gain by calibration means is described in the specification of Published Unexamined Patent Application No. 10-312655. Also, techniques of storing variable factors other than gain as a feedforward signal in a table are described in the specifications of Published Unexamined Patent Application No. 9-139032 and U.S. Pat. No. 5,859,742.
A feedforward control output (feedforward signal) is given as a current value or a target acceleration of a voice coil motor (VCM), for example. To achieve faster step response, a feedforward signal preferably includes certain high-frequency components. Mechanical systems for head seek including an arm, etc., however, ordinarily have high-frequency resonance mode. If a signal which can resonate at the resonance frequency of such a mechanical system, corresponding mechanical vibration is increased, which results in an increase in seek time. Therefore there is a need to produce a control output having no resonance frequency. As a control output production means satisfying this requirement, techniques of limiting high-frequency components by using a sine function are disclosed in the specifications of U.S. Pat. No. 6,031,684 and Published Unexamined Patent Application No. 2000-123502. Also, methods of performing filtering on an input signal according to the natural frequency of mechanical vibration of a control object are disclosed in the specifications of U.S. Pat. Nos. 4,916,635 and 5,638,267. A technique of producing a feedforward signal by using a polynomial formed as a time function is disclosed in U.S. Pat. No. 5,469,414.
The inventors of the present invention recognized problems described below when the above-described model following control is applied to an actual HDD head positioning system.
First, there is a problem of a reduction in seek speed due to a modeling error. That is, while high-speed time response is required of the seek operation of HDDs, the feedback loop response currently achieved is not enough for following high-speed seek operation. The highest operating frequency of a feedback control system is generally determined by the zero cross frequency of the open loop. However, components exceeding this zero cross frequency exist in the frequency components of the seek operation of currently available systems. In general, there is a non-coincidence (modeling error) between a control model and an actual control object, and the modeling error results in an error of the controlled position of a head from a target position. An error due to a delay in the response of the feedback control system cannot be suppressed in a sufficiently short time, so that an error of the head position remains at the time of settling. The time required for the head to complete following a track (seek time) is thereby increased. A modeling error occurs necessarily since individual actuators vary in gain and phase or vary depending on operating environments, or for other reasons. To optimize control in the model following method, it is important in practice to consider the way in which a control model is identified and compensated.
Several known techniques may be used as a solution to this problem. For example, the above-mentioned techniques described in the specifications of U.S. Pat. No. 6,031,684, Published Unexamined Patent Application Nos. 10-312655 and 9-139032, and U.S. Pat. No. 5,859,742 may be used. Each of these techniques, however, lacks consideration of a complicated modeling error other than the gain of an actuator such as a phase delay, and is unsatisfactory in terms of a response characteristic at the time of settling.
Second, there is a problem relating to generation of feedforward signal which does not cause mechanical vibration. It is apparent that each of the techniques described in the specifications of U.S. Pat. No. 6,031,684, Published Unexamined Patent Application No. 2000-123502, and U.S. Pat. Nos. 4,916,635 and 5,638,267 is effective in limiting resonance of the seek mechanism. However, the techniques described in the specifications of U.S. Pat. No. 6,031,684 and Published Unexamined Patent Application No. 2000-123502 do not produce an optimum feedforward control output for high-speed seek operation in a case where vibration due to mechanical resonance is inherently small or there is a limit to the maximum VCM current value. This is because it is known that in an ideal case where there is no mechanical resonance, a seek operation based in so-called bang-bang control is theoretically highest in speed.
On the other hand, the techniques described in the specifications of U.S. Pat. Nos. 4,916,635 and 5,638,267 use a filter capable of attenuating only a resonance frequency, enable inclusion of high-frequency components in the signal, and are, therefore, probable to achieve a high-speed seek operation even if there is a limit to the maximum VCM current value. However, this method can be applied only to cases where the characteristics of a control object are known and the reverse characteristics can be obtained, and cannot be said to be always satisfactory in terms of robustness if the possibility of variation in resonance frequency depending on actual individual units is considered. That is, this method characterized by attenuating a signal component only at a particular frequency to avoid mechanical resonance does not ensure the desired performance in a case where an individual unit has a resonance frequency different from that corresponding to the predetermined filter characteristic.
Each of the above-described techniques relating generation of a feedforward signal entails the problem of an increase in the amount of calculation for generating the signal. In particular, in a case where state estimators are used in a control model, the influence of an increase in the amount of calculation is considerable. Calculation functions in an HDD comprise servo calculation for seek control and processing for executing commands from a host. Simplified HDDs recently provided are designed to achieve these calculation functions by using a single microprocessor unit (MPU). If the amount of calculation is increased as a result of adoption of complicated calculation means, there is a possibility of the amount of calculation exceeding the processing capacity of the single MPU to produce a contrary effect of reducing the performance of the HDD.
An object of the present invention is to provide a technique for reducing a modeling error inhering in a control model assumed in model following control, including gain and phase, to such a level as to ensure that the control system can operate with practically no problem relating to the error, in other words, a technique for identifying a practical simple control model and for compensating the model.
Another object of the present invention is to provide a technique for realizing a high-speed step response characteristic even if there is a limit to the maximum VCM current value, and for generating, by a small amount of calculation, a feedforward signal which causes substantially no high-frequency mechanical vibration.
Still another object of the present invention is to provide an HDD to which the above-mentioned practical control model, means for compensating the model, and feedforward signal are applied to reduce the seek time (a time period required to complete track following) or to improve a response performance.
The present invention will be summarized below. According to the present invention, there is provided a rotary recording apparatus comprising a rotary type recording medium on which information is recorded, a head for performing at least reading out information from the recording medium, head drive means for driving the head, head position detection means for outputting data on the current position of the head, and control means for controlling the position of the head relative to the recording medium. The control means includes feedforward control signal generation means for outputting a feedforward control signal in response to input of a target position of the head, a control model which outputs reference position data in response to input of the feedforward control signal, and a feedback controller which outputs a feedback control signal in response to input of the reference position data and negative feedback data. The control means controls the position of the head to be in the target position by inputting the feedback control signal and the feedforward control signal to the head drive means, by setting the negative feedback data to the current position data, and by inputting the difference between the reference position data and the negative feedback data (current position data) to the feedback controller. The control model is compensated so that, in a compensation process state in which compensation is made for an input to the feedback controller as a bias force, and in which the negative feedback is shut off, a sequence of the reference position data and a sequence of the current position data for each control cycle coincide with each other in response to input of the target position.
In the rotary recording apparatus of the present invention, the control model is compensated by being compared with a head position locus obtained by an actual seek operation. That is, an actual head position trajectory is measured to reduce a modeling error, thereby enabling the control model to be suitably matched to control objects varying for each unit. Therefore, the head position error at the time of settling in model-following feedforward control can be reduced to an extremely small value to shorten the seek time required to complete track following. In the present invention, no state estimator is used for the control model and each of the control signals is formed only from a position signal. That is, according to the present invention, speed control is not performed and only position control is performed. Therefore, the control system can be simplified and MPU resources can be effectively utilized. For example, a support logic of a hard disk controller, called servo assist, may be used to reduce the calculation load on the servo system, thus achieving servo control with a single MPU. Since the present invention uses only position control, no mode change from speed control to position control is made. There is no possibility of vibration in high-order mechanical vibration modes caused by a mode change. Therefore the settling time can be reduced.
The control means further includes gain compensation means for compensating the gain of the feedforward control signal. The feedforward control signal gain-compensated by the gain compensation means is input to the head drive means. The control model is formed by an original control model and a compensation filter. The control means also includes means for obtaining, for each control cycle, model position data output from the original control model in response to the target position input in the compensation process state, means for obtaining, for each control cycle, current position data output from the head position detection means in response to the target position input in the compensation process state, means for obtaining a filter to which a sequence of the model position data is input, and from which a sequence of the current position data is output, means for separating the filter into a filter having a DC gain of 1 and a remaining gain, and means for replacing a coefficient of the gain compensation means with a value obtained by dividing the current coefficient by the remaining gain, and for replacing the compensation filter with the filter having a DC gain of 1.
That is, the control model is separated into an original control model and a compensation filter, and gain compensation means for compensating the gain of the feedforward control signal is provided. This arrangement enables compensation of a complicated control model including phase as well as gain. According to the present invention, compensation of the DC gain is performed chiefly by the gain compensation means while compensation of the remaining gain and phase is performed by the compensation filter. According to this method of adjusting separated compensation functions, improved compensation effects can be ensured even if the compensation filter is simplified. Therefore, simpler compensation calculation may suffice.
As the above-described original control model, a rigid-body model which outputs, in response to input of the feedforward control signal, a value obtained by second-order integration can be used. Also, control signal data, etc., after the above-described compensation may be recorded in a table. During ordinary use, the data recorded in the table is read out to be used for control, thereby reducing the calculation load on the MPU.
The feedforward control signal can be obtained by generating a control signal sequence, such as that in bang-bang control output, which causes maximum acceleration and maximum deceleration in the head in response to input of the target position, and by passing the control signal sequence through a low-pass filter having a DC gain of 1. The cutoff frequency of the low-pass filter is set lower than the resonance frequency of the head drive means. A control signal for providing maximum acceleration and maximum deceleration is generated to improve a step response characteristic (seek performance) of the head seek. On the other hand, high-frequency components which cause mechanical vibration of the head drive system can be removed by the low-pass filter to suppers mechanical vibration of the head drive system, so that the settling time and, hence, the seek time can be reduced. That is, the present invention makes it possible to form a control signal capable of rapid response as well as suppressing mechanical vibration.
The control signal sequence in a state of decelerating the head can be formed so as to change smoothly during its attenuation. That is, at a final stage of a response to the target trajectory at which the position of the head is converging to the target position, the force of driving the head is reduced to suppress overshooting beyond the head target position, thereby advancing reducing the settling time the settling time.
Also, in the present invention, the gain of the control model can be compensated according to a change in the gain of the head drive means. More specifically, the square of the feedforward control signal and the product of the feedforward control signal and the feedback control signal are computed in each of control cycles during ordinary seek operation; a cumulative value of each of the square and the product is obtained from the start to the end of the seek operation; the cumulative value of the product is divided by the cumulative value of the square; and the gain of the control model compensated with the value obtained by the division.
The gain is thus compensated to be an optimum value at all times to compensate the difference between the real system and the model system (modeling error) due to a change in operating environment conditions or a change in the disk position. In actuality, the gain of the control object is, for example, the gain of a voice coil motor (VCM), which varies depending on the operating environment and the seek position on the disk. On the other hand, variation in phase of the head drive system due to a change in operating environment conditions or a change in disk position is not considerably large. Therefore, from the viewpoint of these characteristics of a hard disk unit, the present invention comprising adjusting on the gain during use of the disk after the above-described compensation of the control model is considered to be a simpler and high-performance control means suitable for disk units. The above-described gain compensation during use of the disk can be applied to the above-described compensation of the gain of the feedforward control signal performed by the gain compensation means.
In the above-described control means, the control signals (feedforward control signal and feedback control signal) can be input to the control object through a digital filter. The transfer characteristics of this digital filter may be adjusted to enable the control model to be formed more finely. This filter also enables simplification of the above-described compensation filter and is effective in reducing a modeling error over the entire range of seek length.
The above-described rotary recording apparatus of the present invention can also be recognized as a method for realizing its functions in accordance with the present invention.